Automatic test equipment (ATE) refers to an automated, usually computer-driven, approach to testing devices, such as semiconductors, electronic circuits, and printed circuit board assemblies. A device tested by ATE is referred to as a device under test (DUT).
In ATE, timing accuracy refers to applying signals to the DUT that meet predefined timing constraints. For example, the rising edge of a signal may need to reach the DUT within a specified time-frame in order to test the DUT accurately. As the operational speeds of DUTs increase, timing accuracy becomes more critical, since there is typically less tolerance for signal time variations during testing.
The timing accuracy of ATE is dictated by its hardware and by techniques used to calibrate the ATE. For particular ATE, different calibration methods can yield different timing accuracies. Therefore, proper calibration is one way to improve timing accuracy without the often substantial cost of upgrading the ATE's hardware.
Timing accuracy can be measured in different ways. One commonly used calibration standard is called edge placement accuracy (EPA). In EPA, timing events for communication channels of an ATE, such as identification of signal edges, are measured using an external instrument. Discrepancies between measured signal edge timings and predetermined signal edge timings are defined to be the EPA of the ATE. An EPA of +/− 100 ps, or better, is required to test ATEs that operate at speeds of 400 MHz, or higher. To achieve such testing accuracy, two ATE calibration techniques are often used.
One such ATE calibration technique involves calibrating the ATE externally using a tool, such as a robot or cal-fixture. Another ATE calibration technique involves calibrating the ATE internally. This technique, known as time domain reflectometry (TDR), measures an incident signal edge and a reflected signal edge, and calculates a signal path length based on a difference between the two measurements. The signal path length is then used to adjust signal transmission. However, there is significant calibration error associated with TDR, which results mostly from signal degradation of the reflected edge. That is, the signal must travel twice through the signal path (the signal and its reflection must both travel through the signal path), resulting in signal loss and distortion. To counteract this problem, TDR requires high-bandwidth signal paths, such as relays.